Probably because you don't notice the corruption.

That vendor that has lots of technical issues and the manager that is refusing to change. Well, they know each other and the vendor is giving kickbacks for dragging it out as long as possible.

Most of the corruption in government IT contracting is more subtle; its things like government managers developing relationships with contractors and favoring them subtly without kickbacks, simply because they are comfortable and feel better working with them. The people doing it probably mostly feel like they are actually doing a public good and working around red tape, because they genuinely believe that intangibles that wouldn't be weighed properly in the formal process do weigh in favor of the contractor being a better value.

OTOH, yes, the kind with actual kickbacks absolutely exists, too.

I don't even know if I call that corruption. Humans are the key part of business, and relationships with contractors are just a reflection of that. That absolutely happens, near constantly. I don't think of that as corruption, as much as in a world of very difficult success, when someone proves they can be successful, and make you feel confident, it makes logical sense to stick with them.